1. Technical Field
The present description relates to spacecraft communication systems.
2. Related Art
Laser based communication systems (also referred to as “lasercom systems” or “lasercom terminals”) may be used for communication between different spacecrafts (or platforms), for example, a satellite and an airborne, mobile aircraft. The term spacecraft as used herein includes satellites, space shuttles, aircraft and other platforms. For this disclosure, the international space station may also be considered as a spacecraft.
Typically, a lasercom terminal transmits and receives a laser signal to enable communication between two or more platforms (for example, a satellite and an aircraft). Lasercom communication beams are typically narrow and have to be precisely directed at a target platform. For this example, if a satellite has to communicate with an aircraft (target), then the narrow laser beam from the satellite has to be accurately directed at the aircraft.
Typically, lasercom terminals use a two phased process to communicate. The first phase is an acquisition phase and the second phase is a tracking phase. During the acquisition phase, a lasercom terminal (for example, at a satellite) acquires a target (for example, an airborne aircraft). After the aircraft is “acquired”, the second phase is used to track the mobile aircraft (referred to as “tracking”).
Typically, during the acquisition phase, the laser beam has a larger field of view compared to the field of view during the tracking phase. For a geo-synchronized satellite tracking an aircraft platform, the tolerable error in transmitting a laser beam during the acquisition phase may be about 100 micro radians. Since the beam width during the tracking phase is narrower, the tolerable error in directing a laser beam is only about 0.5 micro radians. Hence, for effectively tracking a target, a transmitting lasercom terminal has to accurately point the laser beam at the receiving terminal of the target.
Due to the finite speed of light, it will take a finite amount of time for a transmitted beam from the transmitting lasercom terminal to reach the receiving lasercom terminal. Hence, for accurately pointing the transmitting beam at the target, the apparent angle or point-ahead angle between the transmitting terminal and the receiving terminal should be included in a pointing command for the transmitted beam.
To accurately compute the point-ahead angle, the transmitting lasercom terminal should have accurate knowledge of the relative velocity of the receiving lasercom terminal at a mobile platform (for example, a mobile aircraft). When the receiving lasercom terminal is on a satellite platform, its velocity may be determined by the ephemeris data of the satellite. However, when the lasercom terminal is on an aircraft (or any other mobile platform), it is difficult to accurately determine the relative velocity because updated aircraft ephermis data may not be available to a satellite at all times and the flight pattern of the aircraft may change. Hence, it is a challenge for a transmitting lasercom terminal to determine a point-ahead angle of a mobile platform, when ephermis data is unavailable.
Therefore, for effective communication with a mobile platform, there is a need for a method and system to efficiently determine a point-ahead angle so that a transmitting lasercom terminal (for example, in a satellite) can accurately point a laser beam at a mobile platform (for example, an aircraft.